In general, steam turbines that are one form of rotating fluid machine include a casing, a rotor rotatably disposed inside the casing, a stator vane cascade disposed at an inner circumferential side of the casing, and a rotor blade cascade provided at an outer circumferential side of the rotor and disposed at an axial downstream side of the rotor with respect to the stator vane cascade. When a working fluid in a main flow passage is passed through the stator vane cascade (more specifically, between stator vanes), internal energy (in other words, pressure energy or the like) of the working fluid is converted into kinetic energy (in other words, velocity energy). That is to say, the working fluid increases in velocity. Thereafter, while the working fluid passes through the rotor blade cascade (more specifically, between rotor blades), the kinetic energy of the working fluid is converted into rotational energy of the rotor. This means that the working fluid acts upon the rotor blade cascade to rotate the rotor.
In some kinds of steam turbines, an annular rotor blade cover is provided at an outer circumferential side of the rotor blade cascade and an annularly grooved section with the rotor blade cover placed therein is formed at the inner circumferential side of the casing. In such a turbine structure, an interspatial flow passage is formed between an outer circumferential surface of the rotor blade cover and an inner circumferential surface of the grooved section in the casing facing the outer circumferential surface. Although a large portion of the working fluid flows along the main flow passage and passes through the rotor blade cascade, a portion of the working fluid is likely to leak as a leakage fluid from the main flow passage into the interspatial flow passage, thus fail to pass through the rotor blade cascade, and consequently make practically no contribution to rotor rotation.
Interspatial flow passages typically have a labyrinth seal to prevent such a leakage flow as described above and enhance turbine efficiency. The labyrinth seal includes a plurality of stages of sealing fins on the rotor side or the casing side, the fins being spatially arranged in an axial direction of the rotor. A seal gap of the labyrinth seal (i.e., a dimension of a clearance reducing portion defined between a distal end of each sealing fin and an area facing the distal end) is limited for purposes such as accommodating any deformation and displacement of members due to thermal expansion or thrust loading. Even when the labyrinth seal is disposed, therefore, a leakage flow from the main flow passage into the interspatial flow passage occurs, which then results in unstable vibration. The fluid force component causing the unstable vibration will be described below with reference to FIG. 14.
FIG. 14 is a sectional view taken along a radial direction of a rotating section 100 to schematically shows an interspatial flow passage 104, the interspatial flow passage 104 being formed between an outer circumferential surface 101 of the rotating section 100 (the outer circumferential surface 101 is equivalent to the outer circumferential surface of the rotor blade cover discussed above) and an inner circumferential surface 103 of a stationary section 102 (the inner circumferential surface 103 is equivalent to the inner circumferential surface of the grooved section in the casing discussed above). The rotating section 100 in FIG. 14 is rotating in a direction indicated by arrow A. In addition, for reasons such as a manufacturing tolerance, gravity, or vibration during rotation, the rotating section 100 is located in an eccentric position denoted by a solid line in FIG. 14, not in a concentric position denoted by a dotted line in the figure, with respect to the stationary section 102. In other words, the rotating section 100 has its center offset from that of the stationary section 102 by the amount of eccentricity, ‘e’. This offset causes the interspatial flow passage 104 to assume circumferential nonuniformity of its lateral dimension D (in other words, its radial dimension between the outer circumferential surface 101 of the rotating section 100 and the inner circumferential surface 103 of the stationary section 102).
A leakage fluid that has flown from a main flow passage into the interspatial flow passage 104 is flowing, for example, in a helical form as indicated by arrow B in FIG. 15. This helical flow can be broken down into an axial velocity component and a circumferential velocity component. The circumferential velocity component and the deviation of the lateral dimension D of the interspatial flow passage 104 cause a nonuniform circumferential pressure distribution P of the interspatial flow passage 104, as shown in FIG. 14. A force that the pressure distribution P exerts upon the rotating section 100 can be resolved into a force Fx applied in an opposite direction (an upward direction in FIG. 14) with respect to a decentering direction and a force Fy (hereinafter referred to as the unstable fluid force) that is applied vertically (a rightward direction in FIG. 14) with respect to the decentering direction. The unstable fluid force Fy causes whirling of the rotating section 100. The unstable vibration of the rotating section 100 occurs when the unstable fluid force Fy is greater than a damping force of the rotating section 100.
A relational formula that uses the unstable fluid force Fy and the amount of eccentricity, ‘e’, is represented as following formula (1). Formula (1) can be obtained by supposing that the rotating section 100 whirls at a speed and that its whirling orbit is a true circle, and omitting an inertial term. In formula (1), ‘k’ denotes a spring constant of the fluid force, ‘C’ a damping coefficient, and ‘C*Ω’ a damping effect of the fluid force associated with whirling.Fy/e=k−C*Ω  (1)
To stabilize the whirling of the rotating section 100 and cause no unstable vibration, formula (1) needs to have a negative value on its right-hand side. Realistically, however, another stabilization element such as a bearing is present. The right-side value of formula (1) does not need to be negative but it is desirable that this value be small. That is to say, it is desirable that the spring constant ‘k’ of the fluid force be small and that the damping coefficient C be large.
As described in Patent Document 1, for example, a conventional technique for reducing the foregoing unstable fluid force is known to reduce a circumferential velocity of a leakage fluid during a flow of the leakage fluid from a main flow passage into an interspatial flow passage. In the conventional technique described in Patent Document 1, for example, a frictional resistance portion is disposed on a side surface of a grooved section of a casing in an interspatial inlet located at an upstream side of the interspatial flow passage.